Analytical formulations for ship-offshore wind turbine collisions

Abstract

Owing to the changing of climate, environmental pollutions and the energy supply are getting short of demand, renewable energy has been gradually emphasized by many of the countries around the world nowadays. The wind energy is one of the highly developed renewable energy which has certain efficiency and potential. Among onshore and offshore wind turbine, offshore is more efficient but also more challenging. One of the most common offshore wind turbine, jacket type, is currently facing some challenges which are the collisions between its foundation and an OSV (Offshore Supply Vessel) or passing ships. Therefore, the crashworthiness of the foundation should be assessed somehow with a certain accuracy.

In order to avoid expensive and time-consuming analysis in preliminary design stage, a simplified analytical method has already been developed which is based on the concept of super-element. The oblique impacted leg (or brace) damage is studied by analyzing two superelements corresponding to horizontal and vertical cylinders. The absorbed energy and impact load of the impacted cylinder in ship-jacket collision issue are able to be predicted rapidly and accurately in preliminary design stage by using this method. Nevertheless, not only the impacted member which absorbs nearly 60% of the total energy is involved but also some of the other members of the jacket foundation. The rear legs are especially on influence as they may be punched by the displacement of the impacted leg through the braces.

The aim of the thesis is to improve the simplified analytical method by developing a new superelement in order to account for the punching phenomena in the super-element method. Based on the concept of virtual work, the external energy is transferred to the internal energy associated with the deformation of the cylinders. The crushing resistance of a punched cylinder is then derived by choosing a realistic displacement field (related to the punching phenomenon) and by calculating the corresponding internal energy.

In addition, numerical solutions from non-linear finite element simulations performed using the finite element code LS-DYNA are used for validating the analytical formulations. The validations are first starting with simple tubular joint and then a real jacket structure is considered. In both cases, good agreement is found between super-element and LS-DYNA simulations.

In conclusion, discrepancy of total absorbed energy between numerical and analytical calculations is still appreciable. In addition to the energy dissipated by crushing of the impacted and the punched members, some part of the striking ship kinetic energy is also dissipated by buckling of the braces and shearing of the legs near the mud line on occasion. Therefore, these tasks are proposed to be studied further for a more accurate assessment of the total absorbed energy of the jacket.